Self-cooling type motors are usually equipped with an individual fan motor at the rear end thereof, but a spindle motor for rotatively driving the spindle (tool spindle) of a machine tool cannot be satisfactorily cooled by a self-cooling fan, because the spindle motor operates over a wide speed range of from a low speed to a high speed.
Accordingly, as shown in FIG. 3, the conventional spindle motor is provided, on the outer circumference of a stator 1, with an oil jacket 3 having helical groove 31 formed in the outer circumference thereof and covered with an outer sleeve 4, to form a path extending from an inlet opening 30 formed on the outer sleeve 4 to an outlet opening 32 formed on the outer sleeve 4, and a cooling liquid is passed through the path as indicated by arrows F.sub.1 and F.sub.2 to cool the oil jacket 3.
The conventional oil jacket cooling system cannot efficiently cool the stator, because this is an indirect cooling system which removes heat transferred, from the stator heated by the heat generated by the coils, projecting portions 10 of the to the oil jacket.
Furthermore, the conventional cooling system cannot remove heat generated in the rotor 2 and in end rings 20 attached respectively to opposite ends of the rotor 2 on the drive shaft 5.
The present invention provides a means which eliminates the disadvantages of the conventional motor cooling system, and is capable of removing heat generated in the rotor and the stator coils.